Foundation GPC Polymers and Molecular Weight - Agilent .and Processing Simple isocratic ... Polystyrene

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  • Conventional GPC

    Polymers and

    Molecular Weight

    GPC On Tour, Barcelona,

    28th February 2012


  • What are Polymers?


    Polymers are long chain molecules produced by linking small repeat units

    (monomers) together

    There are many ways to link different types of monomer to form polymers

    Polymers exhibit very different physical properties compared to the monomers,

    dependent on the length of the polymer chains

    The presence of small amounts of very long or very short chains can have

    drastic effects on properties of the material

  • Variations in Polymers


    They can be varied in lots of ways, for example;

    Chemical Structure of Monomer Unit

    3D Structure

    Different Monomer Units

    Length of polymer chains

    Distribution of polymer chain lengths

  • Example 1 - Nitrocellulose


    First synthetic polymer made in the 1890s

    Hard, strong when set, durable when in moulding

    Soon to be renamed gun cotton!

  • Example 2 - Nylon


    New York London (NY-Lon)

    1935 Dupont Chemical Co.

    Replaced silk in military parachutes

    First product was nylon fibred toothbrush

    Tights came in the 1950s

  • Example 3 - Bullet-proof vests Kevlar


    Strong inter-chain linkages make Kevlar

    bullet proof

  • Example 4 DNA, Deoxyribonucleic Acid


    Longest natural occurring polymer

    DNA in lungfish is 36 meters long per cell

    DNA in humans is about 1 meter long per cell

    Double helix, spiral, symmetry

  • Common Polymers


    Polystyrene PS

    Polyethylene PE, HDPE

    Polyvinylchloride PVC, UPVC


  • Molecular Weight


    The molecular weight of a polymer is a way of describing how long the

    polymer chains are

    Each monomer has a molecular weight (often called the formula weight)

    Adding the monomers together to make polymers increases the molecular


    The longer the chains, the higher the molecular weight

  • Effect of Molecular Weight


    For example, lets look at hydrocarbons

    Very short chain hydrocarbons are the

    predominant component of petrol

    liquid at room temperature

    Longer chain hydrocarbons are present

    in various waxes such as candle wax

    soft, pliable and easy to melt

    Polythene is a very long chain

    hydrocarbon tough, strong and very

    resistant to heat and solvents

  • Polymer Molecular Weight Distributions


    Samples of synthetic polymers always contain polymer chains with a range

    of chain lengths

    One way to describe the length of the polymer chains is in terms of an

    average molecular weight, i.e the average of all the chain lengths in the



    Different samples of the same polymer can have the same average chain

    length but very different distributions of chain lengths depending on the

    method of production

    In polymer science it is the molecular weight distribution that is important

  • Molecular Weight Averages by GPC


    Number average Mn

    Weight average Mw

    Z average Mz

    Polydispersity, d = Mw


    Mn can be correlative with

    polymer colligative properties,

    e.g. freezing point depression

    Mw may be correlated with

    properties such as melt


    Mz may be correlated with

    properties such as


    Polydispersity characterises

    the shape of the distribution

  • Defining the Molecular Weight Distribution


    A molecular weight distribution can be defined by a series of average values

    Except Mp, these are various moments of the average of the molecular weights

    of the distribution

    Mp is the molecular weight of the peak maxima

    For any polydisperse peak:


  • Shape of Distributions


    Even for the same type of polymer, each of

    these distributions will describe a polymer that

    behaves differently

    The red and green plots are for low and high

    polydispersity materials

    The blue plot shows a high polydispersity

    material with a additional high molecular

    weight component

    Describing these distributions is not easily,

    especially if they are complex

  • Effect of Polydispersity on a Polymer


    As the broadness of the distribution decreases

    the strength and toughness of the polymer


    However as the broadness of the distribution

    decreases the polymer becomes more difficult

    to process

    GPC provides key information to predict the

    processability and material properties of a


    Strength Toughness Brittleness Melt





    Increasing Mw + + + + + -


    distribution + + - + + +

  • Measuring Molecular Weight


    There are many ways to measure molecular weights

    Examples include osmometry, centrifugation and batch light scattering

    Each of these methodologies gives a single measurement, and average

    molecular weight

    For example, light scattering measures Mw, osmometry measures Mn and

    centrifugation measures Mz

    Although these methods give you a molecular weight, they do not describe a


    Gel permeation chromatography (sometimes called size exclusion

    chromatography) is a method of measuring molecular weights

    The advantage of GPC is that it is a separation technique, and as such it is the

    only common technique that allows the measurement of the molecular weight

    distribution, not just a single average value

  • So where in Chromatography is GPC


    Interactive adsorption, partition, ion exchange, etc

    Non-interactive GPC, SEC, GFC

  • Nomenclature

    There are many ways of measuring the molecular weight

    of a polymer however, there is only one technique that

    measures the molecular weight distribution.

    Gel Permeation Chromatography, GPC (Polymer Industry)

    One technique but multiple acronyms, also known as

    Size Exclusion Chromatography, SEC (Academia)


    Gel Filtration Chromatography, GFC (Protein/Pharma)

  • Liquid




    Detectors, Data


    and Processing

    Simple isocratic system

    Provides solvent flow

    Column separates sample

    Into components

    Detectors measure the

    properties of eluted sample,

    data system provides


    A simple isocratic LC system fitted with a GPC/SEC column is a

    GPC/SEC system!

    Mode of separation only difference to other HPLC methods

    Specialist detectors can be used to determine properties of the

    samples investigated

    Special GPC/SEC software required to perform analysis

    What is a GPC/SEC System?

  • Additional Components Used in GPC


    Concentration detectors

    Differential refractometer (RI)

    Ultraviolet absorbance (UV)

    Evaporative light scattering or mass detector (ELS, EMD)

    Infra-red (IR)

    Molecular weight sensitive detectors


    Light scattering

    Additional systems

    Online degasser


    Column oven

    Additional specific detectors

  • Polymer Molecules in Solution


    GPC is based on the behaviour of polymer molecules in solution

    In the solid state polymers can be considered like spaghetti a confusing mass of intertwined chains

    In solution, polymer molecules are discrete entities

    Due to entropic effects all but the most rigid of polymer chains curls up in solution to form a ball like shape

  • GPC Column Packings


    GPC columns are packing with cross-linked, insoluble beads, typically co-polymers of styrene and divinyl benzene for organic GPC

    These beads have a rigid pore structure that remains intact in the presence of solvent

  • Synthesis of Porous Beads


    High cross-link content gives a rigid, low swelling

    product with a well-

    defined pore structure

  • Permeation of Polymer Molecules


    Polymer coils in solution can permeate the pores on GPC

    packing materials

    Exclusion, partial permeation and total permeation are possible

  • GPC Separation Mechanism


    Polymer is prepared as a dilute solution in the eluent and injected into the system

    The GPC column is packed with porous beads of controlled porosity and particle size

    Large molecules are not able to permeate all of the pores and have a shorter residence

    time in the column

    Small molecules permeate deep into the porous matrix and have a long residence time

    in the column

    Polymer molecules are separated according to molecular size, eluting largest first,

    smallest last

  • Elution Profiles


    As a result of the GPC separation mechanism, polymer molecules elute

    from the column in order of size in


    Largest elute first, smallest elute last The separation is purely a physical